Hydraulic apparatus for construction machines

Abstract

A hydraulic apparatus for a construction machine, which is adapted to prevent the number of revolutions of an engine from increasing when the flow of the fluid discharged by a variable displacement hydraulic pump driven by the engine is cut off. The hydraulic apparatus comprises a proportional solenoid (11) connected to a fuel supply control lever (13) so as to vary the horsepower characteristics of the engine, which are set by the action of a governor (12) based on manipulated variable of the fuel supply control lever (13) when the flow of the fluid discharged by said pump is cut off, and a controller (10) for controlling the proportional solenoid (11). In this apparatus, a decrement (ΔNr) in a target number of revolutions per minute (Nr) of the engine, which can be obtained by subtracting a target number of revolutions per minute (Nrb) of the engine at the desired engine horsepower characteristics (l 2 ) from a target number of revolutions per minute (Nra) at the initial engine horsepower characteristics (l 1 ) is computed, and said proportional solenoid (11) is operated in accordance with this decrement.

We claim: 
     
       1. A hydraulic apparatus for a construction machine wherein at least one variable displacement hydraulic pump driven by an engine is controlled in accordance with equi-horsepower characteristics and with means for determining when the delivery pressure of the pump reaches or exceeds a preset value, control means cuts off the flow of the fluid discharged by the pump wherein means is provided for preventing the number of revolutions of the engine from increasing when the flow of the fluid discharged by the pump is cut off, said means for preventing the number of revolutions from increasing comprising a proportional solenoid connected to a fuel supply control lever of a governor so as to vary the horsepower characteristics of the engine, which are set by the action of the governor based on manipulated variable of the fuel supply control lever, and a controller for controlling the proportional solenoid, the arrangement being such that when the flow of the fluid discharged by the pump is cut off, the controller functions to compute a decrement in a target number of revolutions per minute of the engine, which is obtained by subtracting a target number of revolutions per minute of the engine at desired engine horsepower characteristics from a target number of revolutions per minute of the engine at the initial engine horsepower characteristics, and said proportional solenoid is operated in accordance with this decrement.